One type of well-known reproduction machines (e.g. copiers, etc.) uses a continuous loop of a photoconductor film to transfer the image to be copied onto a sheet of a copy medium. The film is charged and passes through an input section where the desired information (hereinafter “image”) is projected onto the charged film. The film then moves through a developing section where toner is applied to the charged image, and on through an image transfer section where the toner is transferred onto a copy medium. The toner (i.e. image) is then fixed (i.e. fused) to the copy by the application of heat/pressure.
Typically, the copy medium is cut sheets of paper or transparent material (hereinafter referred to as “sheet(s)”). As is recognized in the art, it is extremely important that each sheet be accurately aligned (i.e. registered) relative to the film when an image is transferred from the film onto its respective sheet. That is, if the axis or centerline of a sheet is “skewed” in relation to the film when the image is transferred, the image will be skewed on the sheet, which in turn, can seriously detract from the esthetic quality of the copy. While small angles of skew (i.e. “skew angle”) may be tolerated since they are not readily discernable to the naked eye, larger skew angles (e.g. >about 0.1 degrees) become quite noticeable and result in unacceptable copies for most users.
Even the smaller skew angles are concern in high-quality, duplex printing/copying operations wherein an image is to be copied onto both sides of a sheet. That is, in high quality duplex reproduction machines (e.g. copiers/printers), it is important that the image on one side of a sheet substantially align with the image on the other side of that sheet. For example, in book printing and the like, the margins of the text on one side of a page should align with the margins of the text on the other side of that page so that a reader will not be distracted by the misaligned print which almost always faintly “shows through” unless the sheet material is unusually thick.
In standard book printing and similar operations, aligning the images on both sides of a sheet is typically accomplished, by using precision-cut, rectangular sheets and providing sophisticated registration mechanisms as part of the sheet feed devices. Such techniques, if applied routinely to “on-demand” copiers where the sheets are not always mill-cut would substantially increase the price thereby making such copiers unavailable to a large portion of the market. Accordingly, the proper registration of the sheets, especially in every-day, duplex copy operations, still needs to be addressed.
Several apparatuses have been proposed for registering the sheets in a copier as the sheets are individually fed into the image transfer section of a copier or the like to alleviate skewing of the images on the sheets. For example, see U.S. Pat. No. 5,322,273, issued Jun. 21, 1994, and the references cited and discussed therein. In U.S. Pat. No. 5,322,273, a sheet registration mechanism is disclosed for aligning each sheet during multi-pass, copy operation. The registration mechanism is comprised of two pairs of sensors, spaced on either side of the center line of the sheet, which sense the leading edge of the sheet to start and stop stepper motors which, in turn, operate friction rollers to compensate for the skew of the sheet in relation to its center line.
While these prior-art registration mechanisms have been successful in most applications, they fail to address the problems involved in aligning the images on both sides of a sheet as are present in duplex copying operations. That is, while mechanisms such as that shown in U.S. Pat. No. 5,322,273, are effective in reducing the skew angle of the image on a sheet to one which is normally indiscernible to the naked eye, these mechanisms are not perfect and a small skew angle may remain, even after a sheet has passed through the registration mechanism, especially if the sheet is not perfectly cut.
In duplex copying/printing operations, an image, e.g. text, is copied onto one side and then the sheet is turned over and an image, e.g. text, is copied onto the other. As explained above, it is esthetically important that these images (the effective boundaries thereof) substantially align with each other once copied on a sheet. That is, the images, if skewed at all, should be skewed at the same angle with respect to their respective lead edges so that one side does not produce a distracting “phantom” image with respect to the other during normal viewing.
If a residual skew angle exists after a sheet has been initially registered for copying on a first side and is not compensated for, the skew angle will be repeated on the other side, thereby effectively doubling the amount of skew between the images on the respective sides of the sheet. Again, while the residual skew angle may be small enough not to present any problems when viewing only one side of the sheet, the combination of the residual angles on both the front and the back of the sheet produces a highly, noticeable and usually objectionable phantom profile of images when a duplex copy is viewed from either side.